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Ambient Temperature Work Advances Optical Clock

Photonics.comJan 2013
BRAUNSCHWEIG, Germany, Jan. 3, 2013 — An optical clock with neutral strontium atoms is considered a strong candidate for the definition of a “new” second, thanks to scientists who have measured, for the first time, the most important uncertainty factor: ambient temperature.

The influence of ambient temperature previously could be derived only theoretically, but the technique developed by scientists at Physikalisch-Technische Bundesanstalt (PTB) has reduced the measurement uncertainty by one order of magnitude. The results might spark interest in geodesy and in fundamental physical research, but more importantly, they have left scientists pondering whether fundamental constants are really constant.

Optical clocks are deemed the clocks of the future for several reasons. They could allow the International System of Units (SI) base unit, the second (the most accurate of all SI base units), to be realized even more accurately. Its definition would no longer be based on the interaction of microwave radiation with cesium atoms, but on the interaction of optical radiation with strontium (or other) atoms or ions.

View of the ultrahigh vacuum chamber where strontium atoms are cooled and stored. The blue fluorescent light in the upper third of the window is a cloud of cold strontium atoms (the drop-shaped formation below the blue fluorescent atom beam in the upper part of the vacuum window). Courtesy of PTB.
However, even before the new definition, optical clocks had found use; e.g., in geodesy, they can help determine Earth’s geoid, or sea level position, more accurately than before. They also provide fundamental physicists with long-awaited instruments to detect possible changes in the fundamental constants.

Optical clocks are accurate because optical radiation oscillates extremely fast — considerably faster than microwave radiation, which is currently used in cesium atomic clocks to “produce” the second. The faster the oscillating system of a clock, the finer the time can, in principle, be broken down and, thus, the more stable and accurate the clock becomes.

In an optical strontium clock, a cloud of neutral atoms is cooled in two steps by means of laser radiation until the atoms finally exhibit a speed of only a few centimeters per second. An “optical lattice” ensures that the atoms are trapped and can virtually no longer move. However, strontium atoms react strongly to changes in the ambient temperature; causing their atomic levels to shift energetically and the clock to become inaccurate. This is the clock’s highest contribution of uncertainty.

To measure this uncertainty with accuracy, the PTB team considerably amplified the effect by using a static electric field instead of the alternating electromagnetic field of blackbody radiation. They constructed a special parallel-plate capacitor whose electric field is known with an accuracy of a few tens of parts per million. In the experiment, the distance between the two plates, which amounted to 0.5 cm, may vary by only a few hundred nanometers over its length of 7 cm; the same applies to the accuracy of the distance.

With the parallel-plate capacitor, the researchers measured the influence of the electromagnetic fields on the two decisive eigenstates in the strontium atom. They determined its uncertainty contribution to the total measurement uncertainty to 5 × 10–18. The next frequency measurements of the clock as a whole could lie well below the previously attained 1 × 10–16.